Implantable Medical Device with Single Coil for Charging and Communicating

ABSTRACT

A combination charging and telemetry circuit for use within an implantable device, such as a microstimulator, uses a single coil for both charging and telemetry. In accordance with one aspect of the invention, one or more capacitors are used to tune the single coil to different frequencies, wherein the coil is used for multiple purposes, e.g., for receiving power from an external source and also for the telemetry of information to and from an external source.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. applicationSer. No. 12/099,474, filed Apr. 8, 2008, which was a divisional of U.S.application Ser. No. 11/047,052, filed Jan. 31, 2005 (now U.S. Pat. No.7,379,775), which was a continuation of U.S. patent application Ser. No.10/679,621, filed Oct. 6, 2003 (now U.S. Pat. No. 6,856,838), which wasa continuation of U.S. application Ser. No. 09/799,467, filed Mar. 5,2001 (now U.S. Pat. No. 6,631,296), which application claimed thebenefit of U.S. Provisional Application Ser. No. 60/189,992, filed Mar.17, 2000. Priority is claimed to each of these applications, and each isincorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

The present invention relates to implantable medical devices, and moreparticularly to a voltage converter for use within an implantablemicrostimulator, or similar implantable device, that uses an RF-poweringcoil instead of capacitors to provide a voltage step-up and step-downfunction. The present invention also relates to an implantable medicaldevice with a single coil that can be used for both charging and thetelemetry of information, potentially at two distinct frequencies.

Many implantable medical devices, such as neural stimulators, sensors,and the like, utilize a battery as a primary source of operating power.Other types of implantable devices, such as cochlear stimulators, relyon the presence of an alternating magnetic field to induce an ac voltageinto the implantable device, where the induced voltage is thereafterrectified and filtered in order to provide the primary operating powerfor the device. In both types of devices—a battery-powered device or anRF-powered device—there is a frequent need to derive other operatingvoltages within the device from the primary power source. That is, thereis a frequent need to step up the voltage of the primary power source toa higher voltage in order to, e.g., generate a high stimulation currentor for some other purpose. Similarly, in some devices, there is also afrequent need to step down the voltage of the primary power source to alower voltage for use in certain types of circuits in order to, e.g.,conserve power.

In order to perform the voltage step-up or step-down function, it isknown in the art to use a charge-pump voltage converter circuit. Chargepump circuits typically rely on a network of capacitors and switches inorder to step up and step down a primary voltage source. For example, inorder to step up a primary voltage source, a network of, e.g., fourcapacitors, may be connected in parallel through a switching network andmaintained in the parallel connection configuration until each capacitorcharges to the voltage of the primary power source. The voltage of theprimary power source is, e.g., the battery voltage (where a battery isused as the primary power source). Once thus charged, the capacitors areswitched so that they are connected in series, thereby effectivelycreating a voltage across the series connection that is four times thevoltage of the primary voltage source. The charge associated with thishigher voltage may then be transferred to another capacitor, e.g., aholding capacitor, and this process (or charging parallel-connectedcapacitors, switching them in series, and then transferring the chargefrom the series connection to a holding capacitor) is repeated as manytimes as is necessary in order to pump up the charge on the holdingcapacitor to a voltage that is four times as great as the voltage of theprimary power source.

While charge-pump circuits have proven effective for performing step upand step down functions, such circuits require a large number ofcapacitors, which capacitors may be quite large and bulky. Charge pumpcircuits that use large numbers of bulky capacitors are not well suitedfor implantable medical devices that must remain very small. Moreover,charge pump circuits tend to be relatively slow and inefficient inoperation. What is needed, therefore, is a voltage converter circuitthat is able to perform the step up or step down function, efficiently,quickly, and without having to rely on the use of a large number ofbulky capacitor/s.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providing avoltage converter for use within small implantable electrical devices,such as a microstimulator, that uses a coil, instead of capacitors, toprovide the voltage step up and step down function. The output voltageof such converter is controlled, or adjusted, through duty-cycle and/orON/OFF modulation. Hence, good efficiencies are achieved for virtuallyany voltage within the compliance range of the converter.

In accordance with one aspect of the invention, applicable toimplantable devices having an existing RF coil through which primary orcharging power is provided, the existing RF coil is used in atime-multiplexing scheme to provide both the receipt of the RF signaland the voltage conversion function. This minimizes the number ofcomponents needed within the device, and thus allows the device to bepackaged in a smaller housing, or frees up additional space within anexisting housing for other circuit components. The result is animplantable device having a voltage converter that may be much smallerand/or more densely packed than prior implantable devices.

In accordance with another aspect of the invention, the voltage up/downconverter circuit is controlled by a pulse width modulation (PWM) and/orON/OFF modulation (OOM) low power control circuit. Such operationadvantageously allows high efficiencies over a wide range of outputvoltages and current loads.

According to another aspect of the invention, an implantable devicecontaining a coil is provided, wherein the coil is used for multiplepurposes, e.g., for receiving power from an external source and also aspart of a voltage conversion circuit. Alternatively, or conjunctively,the coil may be used for receiving command information from an externalsource and also as part of a voltage conversion circuit.

It is thus a feature of the present invention to provide a voltageconverter circuit for use within an implantable device, e.g., such as animplantable microstimulator or similar type of neural stimulator, thatis compact, efficient, and provides a wide range of output voltages andcurrents.

It is a further feature of the invention to provide a voltage convertercircuit that avoids the use of a network of capacitors switched betweenparallel and series, or other, configurations in order to provide thestep up and step down voltage conversion function.

According to another aspect of the invention, an implantable devicecontaining a single coil as well as one or more capacitors that are usedto tune the coil to different frequencies is provided, wherein thedifferent frequencies can be used to receiving power from an externalsource and for the telemetry of information to and from an externalsource.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a block diagram of an implantable stimulator system;

FIG. 2 is a sectional schematic diagram that illustrates one type ofimplantable microstimulator within which the present invention may beused;

FIG. 3 is a functional block diagram of a typical implantablestimulator;

FIG. 4 illustrates a type of fly back converter circuit that may be usedto step up the voltage of a power source without the use of a switchedcapacitor network;

FIG. 5 is a waveform diagram that defines what is meant by Aduty cycle@for purposes of the present application;

FIGS. 6A-6E illustrate simplified schematic diagrams of circuits thatmay be used in accordance with the present invention to respectivelyachieve the following implantable-device functions: voltage step up(FIG. 6A); voltage step down (FIG. 6B); energy reception (FIG. 6C); datareception (FIG. 6D); and data transmission (FIG. 6E);

FIG. 7 is a simplified schematic diagram that illustrates a voltageconverter circuit made in accordance with the present invention thatselectively performs the five implantable-device functions illustratedin FIGS. 6A-6E; and

FIG. 8 is a table that defines the operating state of the variousswitches M1′, M2, M3, M4 and M5 utilized in the circuit of FIG. 7 inorder to select a desired operating mode for the circuit shown in FIG.7.

FIG. 9 is a simplified schematic diagram that illustrates amicrostimulator with a single coil in accordance with one aspect of thepresent invention that can selectively charge or telemeter informationto and from an external source, wherein the charging and telemetry mayoccur at different frequencies.

FIG. 10 is a table that defines the operating state of the variousswitches M6, M7, M8 and M9 utilized in the circuit of FIG. 9 in order toselect a desired operating mode for the circuit shown in FIG. 9.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

The present invention relates to a particular type of voltage converterthat may be used within an implantable medical device, such as animplantable stimulator, sensor, pump or other type of medical deviceproviding a desired medical function. The invention will be describedbelow in terms of an implantable stimulator, but it is to be understoodthat the invention may be used within many different types ofimplantable devices.

To better understand the environment in which the invention is intendedto be used, it will first be helpful to review a typical implantablestimulation system. Hence, with reference to FIG. 1, a block diagram ofa representative implantable stimulator system 10 is illustrated. Thesystem 10 includes an implant device 20, implanted under the skin 18,coupled to an external control unit 12 through implanted coil 22 andexternal coil 15. The external coil 15 is typically carried in a housing14 connected to the external control unit 12 via flexible cable 13. Anexternal power source 16, which may be, e.g., a rechargeable orreplaceable battery, provides operating power for the external controlunit. The external power source 16 may also provide operating power forthe implant device 20 through the link provided through the coils 15 and22, either continuously or on an intermittent basis. Intermittent poweris provided, e.g., such as when the implant device includes areplenishable power source, such as a rechargeable battery, and thebattery is intermittently recharged.

The implant device 20, when functioning as a stimulator, includes aplurality of electrodes 24 a and 24 b connected to the implant device 20via conductive leads or wires 23 a and 23 b, respectively. Theelectrodes 24 a and 24 b are typically implanted near body tissue ornerves 26 that are to be stimulated.

In operation, the system 10 functions as follows: The implant device 20and electrodes 24 a and 24 b are implanted in the desired location underthe patient's skin 18. It should be noted that while the implant coil 22is shown separate from the implant device 20 in FIG. 1, the coil 22 istypically mounted to or housed within the same hermetically-sealed caseused to house the electronic circuitry associated with the implantdevice 20. Once implanted, power and/or control data, e.g., programmingdata, is transferred to the implant device from the external controlunit 12 via electromagnetic coupling between the implant coil 22 and theexternal coil 15. Once thus controlled or programmed, the implant device20 operates as directed by the control signals received, or as steeredby the program data stored therein, to generate electrical stimulationpulses for delivery to the tissue 26 via the electrodes 24 a and 24 b.

Some implant devices 20 do not contain an implanted power source, andsuch devices must thus receive their operating power continuously fromthe external control unit. Other implant devices 20 do contain animplanted power source, e.g., a rechargeable battery, and such devicesthus receive their operating power from the implanted power source.However, on a regular or periodic basis, such devices must have theimplanted power source replenished, e.g., have the battery recharged.Such recharging occurs via a link with the external control unit 12, orequivalent device, through the coils 22 and 15.

FIG. 2 shows a sectional schematic diagram of one type of implantablemicrostimulator 30 within which the present invention may be used. Themicrostimulator device 30 includes electrical circuitry 32 housed withina hermetically-sealed case 34. At each end of the case 34 are electrodes36 a and 36 b. These electrodes 36 a and 36 b are electrically connectedto the electrical circuitry 32 via conductors, e.g., wires, 37 a and 37b, respectively, and appropriate feed-through conductors 38 a and 38 bthat pass through the wall of the hermetically-sealed case 34.

The advantage of the microstimulator device 30 is that it is very small,and can typically be easily implanted at the desired implant locationthrough the lumen of a hypodermic needle, or other cannula. Oneembodiment of a microstimulator is disclosed, e.g., in U.S. Pat. No.5,324,316, incorporated herein by reference. One method of making such amicrostimulator is disclosed, e.g., in U.S. Pat. No. 5,405,367, alsoincorporated herein by reference.

To better appreciate the advantages offered by the present invention,reference is next made to FIG. 3 where there is shown a functional blockdiagram of a typical implantable stimulator 40. As seen in FIG. 3, thestimulator 40 includes electronic circuitry that performs the followingfunctions: an energy receiver 42, a data receiver 44, a power source 46,a control circuit 48, a voltage converter 50, a pulse generator 52, anda back telemetry circuit 54. An implanted coil 56 is connected to boththe energy receiver 42 and the data receiver 44 and provides a meansthrough which power and data signals may be received by the stimulator40. Another coil 58, which in some embodiments may comprise the same, ora portion of, the coil 56, is connected to the back telemetry circuit54, and provides a means through which back telemetry data may be sentto an external receiver. Such an external receiver may be included, forexample, within the external control unit 12 (FIG. 1). All of theabove-described elements of the stimulator 40 are housed within anhermetically-sealed housing or case 60, thereby allowing the stimulator40 to be implanted within body tissue.

External to the housing 60, but still adapted to be implanted withinbody tissue, is a plurality of electrodes 62 a, 62 b. Electricalconnection with the plurality of electrodes 62 a, 62 b is establishedthrough a plurality of wire conductors 63 a, 63 b (which may be includedwithin a single implantable lead body, as is known in the art) which arerespectively connected to a plurality of feed-through connectors 64 a,64 b that pass through the hermetically-sealed wall of the case 60. Thepulse generator 52 is electrically coupled to the plurality offeed-through connectors 64 a, 64 b on the inside of the case 60.

In operation, an RF signal (represented in FIG. 3 by the wavy arrow 66)is received through coil 56. Typically, the RF signal comprises amodulated carrier signal. The carrier signal is rectified in the energyreceiver 42 and provides charging power for the power source 46. Thecarrier signal is demodulated in the data receiver 44 and the data thusrecovered provides control and/or programming data to the controlcircuit 48. The control circuit 48, typically a microprocessor, includesmemory circuitry (not shown) wherein programming and/or control data maybe stored. Based on this programming and/or control data, the controlcircuit 48 drives the pulse generator circuit 52 so that it generatesand delivers electrical stimulation pulses to the patient throughselected groupings of the plurality of electrodes 62 a, 62 b.

In the process of generating the electrical stimulation pulses, whichtypically vary in amplitude as a function of the control and/orprogramming data, and in order to conserve power, it is necessary toprovide a high level supply voltage to the pulse generator circuit 52.For example, if the impedance between electrodes 62 a and 62 b is 1000ohms, and if a stimulation current pulse having a magnitude of 10 ma isdesired, a voltage of 10 volts must be present at the electrodes 62 aand 62 b (Ohms law: voltage=current×impedance). This means that anoutput voltage VO of at least 10 volts must be present at the output ofthe pulse generator circuit 52. In turn, this means that a supplyvoltage VC, provided to the pulse generator circuit by the voltageconverter 50, must be greater than 10 volts, e.g., 12 volts or more dueto losses within the pulse generation circuit. Hence, the voltageconverter circuit 50 is typically used in a stimulator 40 to step up thepower source voltage VS, e.g., the battery voltage, to a level suitablefor use by the pulse generator circuit 52. The power source voltage VSis typically a low value, e.g., 2 or 3 volts. Hence, in a typicalstimulator device 40, such as the one shown in FIG. 3, the voltageconverter circuit 50 is needed to boost, or step up, the source voltageVS from its relatively low value to a higher level VC as needed by thepulse generator circuit 52. Unfortunately, in order to provide such astep-up function, bulky and numerous circuit components, such as thecapacitors used in a switched capacitor network, and/or transformers,must be employed.

The difference between the supply voltage VC and the output voltage VOmay be referred to as the compliance voltage. In an ideal pulsegenerator circuit 52, the compliance voltage is kept as low as possiblebecause the power dissipated in the pulse generator circuit (which isgenerally considered as wasted or lost power because it does notrepresent power delivered to the tissue) is proportional to the squareof the compliance voltage. In practice, the compliance voltage cannotalways be minimized because the current delivered through the electrodes62 a and 62 b to the body tissue varies over a wide range; and hence thecompliance voltage must also vary over a wide range.

In some implantable stimulators 40, in order to conserve the amount ofpower dissipated by the stimulator, the voltage converter circuit 50 isused to adjust the supply voltage VC, typically to provide a smallnumber of discrete levels of supply voltage, as a function of thecurrent to be delivered in the stimulation pulse. For example, a typicalvoltage converter circuit 50 may provide one of four different supplyvoltages VC to the pulse generator circuit 52, e.g., a VC of 2.5, 5.0,7.5 or 10 volts, as a function of the programmed amplitude of thestimulation pulse that is to be delivered to the tissue. An implantablestimulator having such a feature is described, e.g., in U.S. Pat. No.5,522,865, incorporated herein by reference.

It is thus seen that the voltage converter circuit 50 performs a veryimportant function within the implantable stimulator 40. Unfortunately,however, the voltage converter circuit 50 represents additionalcircuitry that requires bulky circuit components, which takes up neededand valuable space within the case 60, and much of which also consumesadditional power. Further, most voltage converter circuits 50 tend to bevery inefficient. That is, a capacitor charge pump circuit, for example,typically may operate at efficiencies that may be less than 50%. Thus,for most stimulators, e.g., of the type shown in FIG. 2, space and powerconsiderations are paramount to the design of the stimulator.

The present invention advantageously provides circuitry for use withinan implantable stimulator device that performs the voltage conversionfunction using fewer and less bulky components. This frees up valuablespace within the case of the stimulator that may be used for otherfunctions (or allows the case to be smaller), and consumes less powerthan has heretofore been achievable. Additionally, the present inventionprovides a circuit that performs multiple functions, thus allowing fewercircuit components to be used within the stimulator design, therebypermitting the overall stimulator design to be smaller or more compact.

Turning next to FIG. 4, a type of fly back converter circuit isillustrated that may be used to step up the voltage of a power sourcewithout the need for a switched capacitor network. The fly back circuitshown in FIG. 4 includes an inductor or coil L1 having one end connectedto a power source 70. The other end of the coil L1 is connected to afirst circuit node 72. A switching transistor M1 is connected betweenthe first node 72 and ground. The transistor M1 has a gate terminal 73connected to a duty cycle control circuit 74. When the transistor M1 isturned ON, through application of a signal to its gate terminal 73, node72 is effectively switched to ground potential through a very lowimpedance path. When transistor M1 is turned OFF, through absence of asignal applied to its gate terminal 73, it represents a very highimpedance path, and thus effectively maintains node 72 disconnected fromground.

Also connected to node 72 of the fly back circuit shown in FIG. 4 is thecathode side of diode D1. The anode side of diode D1 is connected to anoutput node 75. An output capacitor C1 is connected between the outputnode 75 and ground. A load, represented in FIG. 4 by phantom resistorRL, is also connected between the output node 75 and ground.

Still with reference to FIG. 4, the duty cycle control circuit 74applies a pulsed signal to the gate of transistor M1, therebyeffectively turning transistor M1 ON and OFF as controlled by the pulsedsignal. For example, a high voltage applied to the gate of M1 may turnM1 ON (provide a low impedance path between node 72 and ground), and alow voltage applied to the gate of M1 may turn M1 OFF (provide a highimpedance path between node 72 and ground). A sequence of high and lowvoltages may be applied to the gate 73 of transistor M1 throughapplication of a pulsed signal 81 generated by the duty cycle controlcircuit 74. When a pulse is present, the voltage is high, and thetransistor M1 is turned ON. When a pulse is not present, the voltage islow, and the transistor M1 is turned OFF.

The ratio of time when the pulse is high to the total cycle time isknown as the “duty cycle”. The duty cycle is defined as shown in FIG. 5.As seen in FIG. 5, a pulsed signal 81 comprises a train of pulses 80.Each pulse 80 comprises a high voltage for a period of time T2 and a lowvoltage for a period of time T3. The total cycle time T1 is equal to T2plus T3. Duty cycle is typically defined as a percentage and is computedas the ratio of T2/T1 or T2/(T2+T3). The duty cycle may thus vary from0% when T1=0, to 100% when T1=T2.

The operation of the fly back circuit of FIG. 4 is known in the art.Basically, when transistor M1 is turned ON, during time period T2,circuit node 72 is connected to ground, which connects one side of thecoil L1 to ground. This connection of one side of the coil L1 to groundcauses an electrical current to start to flow from the power source 70through the inductor coil L1. As soon as T2 ends, however, and for theremaining time T3 of the total cycle time T1, the node 72 floats (is notconnected to ground), which causes the voltage at node 72 to step up toa high value (higher than the voltage of the power source VS, aselectrical current continues to flow through coil L1, through diode D1,to charge capacitor C1. Thus, during time T2, current starts to flowthrough the coil L1, which causes electromagnetic energy to be stored inthe coil. During time T3, this energy is transferred to capacitor C1,thus charging C1. Eventually, typically over several cycles, C1 ischarged up to a voltage that is higher than the power source voltage VS.Capacitor C1 is blocked from discharging to ground through transistor M1by diode D1 when MI is turned ON during time T2. The stored charge heldon capacitor C1 thus provides an output voltage VOUT (greater than VS)that causes an output current IO to flow through the load resistor RL.

The magnitude of the output voltage VOUT and output current IO mayadvantageously be controlled by adjusting the duty cycle of the signal81. A higher duty cycle causes both VOUT and IO to increase, whereas alower duty cycle causes VOUT and IO to decrease. Because the duty cycleis adjusted by controlling the pulse width (T2) of the pulses 80, theduty cycle control circuit 74 may also be referred to as a pulse widthmodulator circuit.

Still with reference to FIG. 4, it should also be noted that feedbackmay optionally be employed to better control and regulate the outputvoltage VOUT. That is, a sensing circuit 76A may be used to monitor theoutput voltage VOUT, and to compare the sensed output voltage to eithera reference voltage VREF and/or a programmed reference signal PROG(which typically is presented to the sensing circuit 76A as a digitalsignal). The sensing circuit 76A generates a difference signal, onsignal line 76C, representing the difference between the sensed outputvoltage VOUT and the reference voltage VREF and/or PROG. This differencesignal controls a gate control circuit 76B, which modulates the gate oftransistor M1 so as to drive the difference signal to zero.

Turning next to FIGS. 6A-6E, additional simplified schematic diagrams ofcircuits are illustrated that may be used in accordance with the presentinvention to achieve desired functions. More particularly, a voltagestep up function may be achieved using the circuit shown in FIG. 6A; avoltage step down function may be achieved using the circuit of FIG. 6B;an energy reception function may be achieved using the circuit of FIG.6C; a data reception function may be achieved using the circuit of FIG.6D; and a data transmission function may be achieved using the circuitof FIG. 6E. Advantageously, many of the components used in the circuitsof FIGS. 6A-6E may be the same. Common reference numerals are used todenote the components that may be the same. A brief explanation of eachof these functions will next be described.

FIG. 6A depicts a circuit that performs a voltage step up function. Thiscircuit is substantially the same as the circuit previously described inconnection with FIG. 4, except that the load resistance RL is not shown.However, it is to be understood that a load resistance may be present.It should also be understood that whereas FIG. 4 shows a duty cyclecontrol circuit 74 controlling switch M1, FIG. 6A shows a PWM (pulsewidth modulation) control circuit 74′ controlling switch M1. Thesecircuits perform the same function (turning switch M1 ON or OFF) and,for purposes of the present invention, are substantially the same.

FIG. 6B depicts a circuit that performs a voltage step down function. Asseen in FIG. 6B, a coil L1 is connected between circuit nodes 75 and 76.Node 75 represents the output node of the circuit whereon the outputvoltage VOUT is present. Capacitor C1 is connected between node 75 andground. The anode side of a diode D2 is connected to node 76, while thecathode side of diode D2 is connected to ground. One leg of a transistorswitch M2 is connected to node 76, while the other leg of transistorswitch M2 is connected to the power source 70 at node 77. A gateterminal 78 of transistor M2 is connected to pulse-width modulation(PWM) control circuit 74″.

FIG. 6C shows a circuit that receives energy from an external source.The energy receive circuit shown in FIG. 6C includes a coil L1 having acapacitor C2 connected in parallel with the coil L1, with one side ofthe parallel connection being grounded. The coil L1 and capacitor C2comprise an “LC” circuit that is tuned to the frequency of an incomingRF signal 83 (represented in FIG. 6C by a wavy arrow). Diode D1 isconnected between output node 75 and the other side of the L1-C2parallel connection, with the cathode of D1 being connected to node 75.Capacitor C1 is connected between output node 75 and ground.

In operation, the circuit shown in FIG. 6C receives the incoming RFsignal 83 through coil L1, tuned to the frequency of the signal 83 bycapacitor C2. Diode D1 rectifies the signal, storing the positive halfcycles of the received signal 83 on capacitor C1. The voltage thusdeveloped on capacitor C1 functions as an output voltage VOUT for usewithin the implant device.

Next, in FIG. 6D, a data receiver circuit is illustrated. Such datareceiver circuit includes coil L1 connected in parallel with variablecapacitor C3. A modulated RF signal 88′ is received through the coil L1.The value of C3 is adjusted, as required, so that the L1-C3 circuit istuned to the frequency of modulation applied to the incoming RF signal88′. Node 72′, which represents an output node of the L1-C3 circuit, isconnected to the input of an amplifier U1. The output signal provided bythe amplifier U1 comprises a Data Out signal that reflects themodulation applied to the incoming modulated RF signal 88′.

Turning to FIG. 6E, a simple data transmitter circuit is depicted. Thedata transmitter circuit includes a coil L1 connected in parallel withan adjustable variable capacitor C3. One side of the L1-C3 parallelconnection is connected to a power source 70. The other side of theL1-C3 parallel connection, identified as node 72′ in FIG. 6E, isconnected to the anode of diode D3. The cathode of diode D3 is connectedthrough a switch transistor M3 to ground. The gate terminal of switch M3is driven by a “Data Mod” (data modulation) signal. Thus, in operation,when switch M3 is closed, a current is drawn through the L1-C3 parallelcircuit. When switch M3 is open, no current is drawn through the L1-C3parallel connection. The on-off current flow through the L1-C3 parallelconnection causes a varying current to flow through coil L1 ascontrolled by the on-off pattern of the Data Mod signal. This currentflow, as is known in the art, induces a varying magnetic field, which inturn causes an RF signal 89 to be radiated, or transmitted, from coilL1.

Thus it is seen that the circuits illustrated in FIGS. 6A-6E provide thefunctions of voltage step up (FIG. 6A), voltage step down (FIG. 6B),energy reception (FIG. 6C), data reception (FIG. 6D), and datatransmission (FIG. 6E). All of these functions are typically requiredwithin an implantable stimulator device (FIG. 3).

In order to perform the functions provided by the circuits shown inFIGS. 6A-6E, while at the same time reducing the number of componentsneeded for each function, and thereby reduce the overall size (and hencevolume, weight and power) of the circuitry that carries out suchfunctions, the present invention advantageously combines all thefunctions performed by the individual circuits shown in FIGS. 6A-6E intoone circuit as shown in FIG. 7. Such combined circuit may be referred toas a “voltage converter using an RF-powering coil”, and is particularlysuited for use within an implantable medical device, such as animplantable neural stimulator.

Advantageously, the combined circuit provided by the present invention,and shown in FIG. 7, uses an RF-powering coil in combination with othercircuit elements to perform the function of receiving RF power from anexternal source. The received RF power may be modulated in order totransmit control data into the circuit. Further, such RF-powering coilmay be used to help transmit data out of the circuit. Significantly, theRF-coil used to receive power, data, and to transmit data, may also beused to selectively convert the received power (i.e., voltage) up ordown in order to make operation of the circuit more efficient.

The circuit of FIG. 7 (i.e., the voltage converter circuit using anRF-powering coil provided by the present invention) includes areceiving/transmitting coil L1′. The coil L1′ includes ends attached tocircuit nodes 72′ and 85, respectively. Node 85, in turn, is connectedthrough transistor switch M1′ to source voltage VS. The coil L1′ furtherincludes a tap point 85′, where there are N2 turns of the coil betweentap point 85′ and node 85, and N1 turns between tap point 85′ and node72′. The coil L1′ thus has a total of N turns, where N=N1+N2.Representative values of N1 are 10 to 100 turns, and for N2 are also 10to 100 turns, and wherein the inductance of coil L1′ is between about 10to 100 microhenries (μH). However, in some embodiments, N1 and N2 mayvary from 1 to 1000 turns, and L1′ may vary between 1 to 1000 μH.

Still with reference to FIG. 7, a series combination of a capacitor C3′and transistor switch M4 is connected between circuit node 72′ and tappoint 85′. Another transistor switch M5 connects the tap point 85′ ofcoil L1′ to ground (node 87). Yet another transistor switch M2 connectsthe tap point 85′ to the source voltage VS.

The cathode end of a diode D2 is also connected to the tap point 85′ ofthe coil L1; while the anode end of diode D2 is connected to ground.

The cathode end of another diode D3 is connected to node 72′. The anodeend of diode D3 is connected through transistor switch M3 to ground(node 87). The anode end of diode D3 is also connected to the input ofsignal amplifier U1.

The cathode end of yet another diode D1 is also connected to node 72′.The anode end of diode D1 is connected to circuit node 75′. A capacitorC1 is connected between node 75′ and ground (node 87). Circuit node 75′is the location where the output voltage VOUT is made available when thecircuit operates in a voltage step up or step down mode. If needed, asuitable voltage clamp circuit 91 may be connected between node 75′ andground in order to prevent the voltage at the output node 75′ fromexceeding some predetermined value.

It is thus seen that the circuit of FIG. 7 includes five transistorswitches, M1′, M2, M3, M4 and M5. The state of these five switches,whether ON, OFF, or modulated with PWM data or signal data, determineswhich circuit function is performed as defined in the table presented inFIG. 8. That is, as seen in FIG. 8, in order for the circuit of FIG. 7to operate in a voltage step up mode, switch M1′ is turned ON, M2 isturned OFF, M3 is modulated with a PWM signal from a suitable duty cyclecontrol circuit (see FIGS. 4 and 5), and both M4 and M5 are turned OFF.Under these conditions, the circuit of FIG. 7 effectively reduces to thecircuit shown in FIG. 6A, with the only difference being diode D3 beingadded in series with switch M3 (which addition does not significantlyalter the operation of the circuit). In such configuration and mode, thelevel of the output voltage VOUT is determined in large part by the dutycycle of the signal applied to the gate of transistor switch M3, asexplained previously.

Similarly, as defined in FIG. 8, for the circuit of FIG. 7 to operate ina voltage step down mode, switch M1′ is turned OFF, switch M2 ismodulated with a PWM signal from a suitable duty cycle control circuit74″ (FIG. 6B), and switches M3, M4 and M5 are all turned OFF. Underthese conditions, the circuit of FIG. 7 effectively reduces to thecircuit shown in FIG. 6B, with the only difference being diode D1connected between nodes 72′ and 75′ (which diode does not significantlyalter the circuit's operation), and only a portion of coil L1′ beingused (i.e., only the turns N1 are used). In such configuration and mode,the circuit performs a voltage step down function, as describedpreviously in connection with FIG. 6B.

As defined in FIG. 8, the circuit of FIG. 7 may also selectively operatein an energy receive mode and a data receive mode by turning switchesM1′, M2 and M3 OFF, and by turning switches M4 and M5 ON. With theswitches in these positions, the circuit of FIG. 7 effectively reducesto the circuit shown in FIG. 6C, and to the circuit shown in FIG. 6D,with the only difference being that just a portion (N1 turns) of thecoil L1′ is used as part of the circuit. In this configuration and mode,the circuit of FIG. 7 thus performs both an energy receive function asdescribed previously in connection with FIG. 6C, and a data receivefunction as described previously in connection with FIG. 6D.

As further defined in FIG. 8, the circuit of FIG. 7 may also selectivelyoperate in a data transmit mode by turning switch M2 OFF, by modulatingswitch M3 with a data signal, and by turning switch M1′ ON. Switch M4may be either OFF or ON depending upon whether capacitor C3′ is deemednecessary to better tune coil L1′ for efficient data transmission. Formany data transmissions, capacitor C3′ should not be needed. Under theseconditions, the circuit of FIG. 7 effectively reduces to the circuitshown in FIG. 6E. Hence, in such configuration and mode, the circuitperforms a data transmit function, as described previously in connectionwith FIG. 6E.

Thus, it is seen that by selectively controlling the state of theswitches M1′, M2, M3, M4 and M5, the circuit of FIG. 7 may operate inany one of five different modes. Some of these modes, e.g., the energyreceive mode and the data receive mode, may operate simultaneously.Others of the modes may be invoked in a time-multiplexed manner, e.g.,with a first mode being followed by a second mode, and with the secondmode being followed by a third mode, as required, depending upon theparticular application at hand. Thus, for example, an energy and datareceive mode may operate as a first mode to allow the device to receiveoperating power (e.g., to recharge a battery) and/or to receive initialprogramming control signals. This first mode may then be followed by asecond mode, e.g., a voltage step up mode, initiated by changing thestate of switches M1′, M2, M3, M4 and M5 as defined in FIG. 8, duringwhich the voltage of the primary power source is stepped up to a voltageneeded by the device in order for it to perform its intended function.Subsequently, as required, a third mode, e.g., a data transmit mode, maybe invoked in order to allow the implant device to transmit data to anexternal receiver.

The component values of the components, i.e., the transistor switchesand capacitors and coil, used in the circuit of FIG. 7 may be readilyascertained by those of skill in the art for a particular applicationand desired RF frequency.

Another example of circuitry for an implantable medical device is shownin FIG. 9. Like the circuitry discussed earlier, the single-coilcircuitry of FIG. 9 can receive and transmit data, and can receive powerfor charging the implantable medical device. Also similar is the tunablenature of the circuitry, which like earlier circuitry can be tuned toone frequency for data transmission and reception, and to another forpower reception. The circuitry of FIG. 9 can also use the single coil toperform the step up and step down features previously discussed,although those aspects of the circuitry are not shown.

The circuit of FIG. 9 includes coil L2. The coil L2 is connected at oneend through transistor switch M6 to power source voltage VS and at theother end through transistor switch M7 to ground. C4 is connected inparallel with coil L2. A series combination of a capacitor C5 andtransistor switch M8 are also connected in parallel to coil L2. Alsoconnected in parallel with coil L2 is the full bridge rectifierrepresented by diodes D4, D5, D6, and D7 for producing DC voltage VOUT.Overall transfer efficiency is expected to be about 15% higher for afull bridge rectifier compared to a single diode, or half-wave,rectifier. However, such other alternatives could also be used toproduce VOUT. A transistor switch M9 is also connected between therectifier circuitry and ground.

DC voltage VOUT is received at storage capacitor C6, which smoothes thevoltage before being passed to charging circuitry 92. Charging circuitry92 is used to charge battery source 93 in a controlled fashion. Ifneeded, a Zener diode D7 or other suitable voltage clamp circuit may beconnected across capacitor C6 to prevent VOUT from exceeding somepredetermined value.

FIG. 10 shows the status of transistor switches, M6, M7, M8 and M9 forthe energy receive, data receive, and data transmit modes. As seen inFIG. 10, for the circuit of FIG. 9 to operate in an energy receive mode,the circuit will turn switches M6, M7 and M9 OFF, and will turn switchM8 ON. Turning M8 ON includes capacitor C5 in parallel with capacitorC4, which, in conjunction with the inductance formed by the coil L2,forms a resonant circuit which is tuned to the frequency f1 of the RFsignal 83 sent from external control unit 12. Such frequency f1 can beapproximately 80 kHz for example.

The circuit of FIG. 9 may also operate in a data transmit mode duringcharging by employing back telemetry known as Load Shift Keying (LSK).As is described in greater detail in U.S. application Ser. No.12/354,406, filed Jan. 15, 2009, during LSK, the impedance of theresonant circuit shown in FIG. 9 is modulated via control of transistorswitch M9, with the transistor's on-resistance providing the necessarymodulation. This change in impedance is reflected back to the coil inthe external control unit 12 which produces the RF signal 83, whichreflection is demodulated at the external control unit 12 to recover thetransmitted data. This means of transmitting data is useful tocommunicate data relevant during charging of the battery 93 in themicrostimulator, such as the capacity of the battery, whether chargingis complete and the external charger can cease, and other pertinentcharging variables.

For the circuit of FIG. 9 to operate in a data receive mode, the circuitwill turn switches M6, M8 and M9 OFF, and will turn switch M7 ON.Turning M8 off excludes capacitor C5 from the resonant circuit, whosetuning is thus governed by coil L2 and capacitor C4. With capacitor C5excluded, the resonant circuit is tuned to a higher frequency f2matching the modulated RF signal 88′ received from the external controlunit 12. (The external control unit 12 that sends the RF data signal 88′can be the same device as, or separate from, the external control unitthat sends the RF power signal 83 to the microstimulator. Even thoughthe external control unit 12 can comprise more than one device, theexternal control unit 12 is however shown as a singular device in FIG. 9for convenience). For example, f2 can comprise approximately 125 kHz,which can comprise a center frequency of the modulated data in the RFsignal 88′. For example, if the RF signal 88′ is modulated in accordancewith a Frequency Shift Keying (FSK) protocol, a logic ‘0’ might comprisea frequency slightly lower than the center frequency (e.g., 121 kHz),while a logic ‘1’ might comprise a frequency slightly higher than thecenter frequency (e.g., 129 kHz); despite the slight difference, thisrange or band of frequencies for the data can be discussed as a singlefrequency. Turning M7 ON grounds the resonant circuit, which provides aninput to the receiver, which demodulates the received data. The receivercan either comprise a differential input as illustrated in FIG. 9, orcan comprise a single-ended non-differential input.

As further shown in FIG. 10, the circuit of FIG. 9 may also operate in adata transmit mode by turning switches M8 and M9 OFF, by modulatingswitch M7 with a data signal, and by turning switch M6 ON. Under theseconditions, the resonant circuit is once again, by virtue of transistorM8 being off, tuned to the higher frequency f2, and will broadcast an RFsignal 89 to the external control unit 12 accordingly, with the energyfor the radiation being supplied from the power source voltage Vs viatransistor M6. The data appearing at transistor M7 will be modulated bya transmitter in accordance with the tuning of the resonant circuit, andso will center around 125 kHz. For example, when transmitting a logic‘0’, the data signal presented to transistor M7 might comprise a 121 kHzsignal, or might comprise a 129 kHz signal when transmitting a logic‘1.’ The transmitter could also couple to transistor M6.

Thus, it is seen that by selectively controlling the state of theswitches M6, M7, M8 and M9, the circuit of FIG. 9 may operate indifferent modes. Such modes may be invoked in a time-multiplexed manner,e.g., with a first mode being followed by a second mode, depending uponthe particular application at hand. Advantageously, tuning allowscharging at a first frequency f1 and data telemetry at a secondfrequency f2. Because lower frequencies, e.g., 80 kHz, have been foundto be more optimal for charging than are higher frequencies due toheating concerns, and because higher frequencies, e.g., 125 kHz, havebeen found to allow for increased bandwidth and higher data rates, thecircuitry of FIG. 9, like other tunable circuits disclosed herein,provides a design that is optimized for both charging and telemetrywhile still using only a single coil for both. A variable capacitorcould be used in place of capacitors C4 and C5 to achieve similartuning.

It is thus seen that the invention described herein provides a voltageconverter circuit for use within an implantable device, e.g., such as animplantable microstimulator or similar type of neural stimulator, thatis compact, efficient, and provides a wide range of output voltages andcurrents.

It is further seen that the invention provides a voltage convertercircuit that avoids the use of a network of capacitors switched betweenparallel and series, or other, configurations in order to provide thestep up and step down voltage conversion function.

It is further seen that the invention provides an implantable devicecontaining a single coil that is tunable to different frequencies forcharging and telemetry.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

1. An implantable medical device, comprising: a single coil, wherein thesingle coil is tunable to wirelessly receive power at a first frequencyfor charging a battery in the implantable medical device, and is tunableto wirelessly receive and transmit data at a second frequency.
 2. Thedevice of claim 1, wherein the single coil is tunable to the first andsecond frequencies by adjusting a capacitance.
 3. The device of claim 1,wherein the first frequency is non-modulated, and wherein the secondfrequency comprises a center frequency for the data.
 4. The device ofclaim 1, wherein the second frequency comprises a center frequency forthe data, and wherein the data is Frequency Shift Key modulated.
 5. Thedevice of claim 1, further comprising a first capacitor in parallel withthe coil, wherein the first capacitor tunes the single coil towirelessly receive and transmit data at the second frequency.
 6. Thedevice of claim 5, further comprising a second capacitor, wherein thesecond capacitor is coupleable in parallel with the coil to tune thesingle coil to wirelessly receive the power at the first frequency. 7.An implantable medical device, comprising; a rechargeable battery; aresonant circuit comprising a single coil, wherein the resonant circuitis tunable to a first frequency to receive power from an external sourceand is tunable to a second frequency to receive data from an externalsource; a first switch coupling a first end of the coil to a powersource voltage; a second switch coupling a second end of the coil toground; a rectifier circuit coupled to the resonant circuit forconverting the received power to a DC voltage for charging the battery;and a receiver coupled to the resonant circuit for decoding the receiveddata.
 8. The device of claim 7, wherein the resonant circuit is tunableto the first and second frequencies using a third switch.
 9. The deviceof claim 7, further comprising a transmitter for transmitting data to anexternal source by controlling the one of the first or secondtransistors.
 10. The device of claim 7, wherein the resonant circuit istunable by adjusting a capacitance coupled to the coil.
 11. The deviceof claim 7, wherein the rectifier circuit comprises a full bridgerectifier.
 12. The device of claim 7, further comprising a third switchfor modulating the impedance of the resonant circuit to transmit data tothe external source from which the resonant circuit receives power. 13.An implantable neurostimulator device, comprising: a housing; arechargeable battery within the housing; a single coil within thehousing, wherein the single coil is tunable to wirelessly receive powerat a first frequency for charging the rechargeable battery, and istunable to wirelessly receive and transmit data at a second frequency;and a plurality of electrodes external to the housing for stimulatingtissue, wherein the electrodes receive power from the rechargeablebattery.
 14. The device of claim 13, wherein the single coil is tunableto the first and second frequencies by adjusting a capacitance.
 15. Thedevice of claim 13, wherein the first frequency is non-modulated, andwherein the second frequency comprises a center frequency for the data.16. The device of claim 13, wherein the second frequency comprises acenter frequency for the data, and wherein the data is Frequency ShiftKey modulated.
 17. The device of claim 13, further comprising a firstcapacitor in parallel with the coil, wherein the first capacitor tunesthe single coil to wirelessly receive and transmit data at the secondfrequency.
 18. The device of claim 17, further comprising a secondcapacitor, wherein the second capacitor is coupleable in parallel withthe coil to tune the single coil to wirelessly receive the power at thefirst frequency.